Prior art document EP-A-0 551 124 discloses a steel cord constituted by a core filament and a plurality of layer filaments around the core filament. The core filament may have a spatial wave form or a planar wave form.
The embodiment with the core filament in a planar wave form is particularly advantageous when used as reinforcement in the breaker or belt layer of a rubber tire. One reason is that if the plane of the wave form is parallel to the rubber ply of the tire, the thickness of the rubber ply does not increase in spite of the fact of the wavy form of the core filament and in spite of the resulting increase in cord diameter. Another reason is that a plane wave form provides an improved rubber penetration compared to a spatial wave form.